To use it, put a “-” in the password box and press the link, this will give you a glass pane with a password box, just write your simple password (Facebook1234 for example) and choose the length, the script will enter the password in the field where you put the “-“.
If you choose Facebook1234 as your simple password the 21 character SHA-1 will be fc1a17377c7ed19872037.

The convert.js script is YUI-compressed, the original one is convert.js.original.
There is also a form based version if you just want the password in cleartext: